Composite synthetic string for tennis racket

ABSTRACT

The invention concerns a composite synthetic string in particular for a tennis racket. The invention is characterised in that the string consists of central monofilaments ( 3 ) surrounded by multifilament yarns ( 2 ), the assembly being impregnated and coated with polyurethane ( 1 ) and is subjected to tractive bending after it has been formed. The invention is useful for tennis strings with novel appearance, with long life span and quick reaction when hitting a ball.

[0001] The present invention relates to a composite synthetic string,which is intended in particular but not exclusively for the stringing oftennis rackets.

[0002] Some composite synthetic strings are already known. Described inFR-A-2 491 098 is a synthetic string with two components: polyamidemultifilament threads and polyurethane, the multifilaments beingintegrated in a matrix of polyurethane. The polyurethane binder, whoseelastic behavior is quite superior to that of polyamide, makes itpossible to obtain, with a particular structure of the string, a highbreaking strength and an average hardness expressed by rapid return ofthe string to its initial position after impact of a ball.

[0003] The present invention relates to using this advantage andconsiderably lengthening the life span of the tennis strings, even whenused with rackets with a rigid frame, while having an originalappearance.

[0004] According to the present invention, the synthetic compositestring, particularly for a tennis racket, having threads of a firstsynthetic material and a binder made of a second synthetic materialimpregnating to the core, connecting and covering said threads, whichare spirally wound, the melting point of the binder being less than thatof the threads and its elasticity much higher, said threads being madeup of multifilaments, is characterized by the fact that at least onemonofilament with two flat sides is arranged in the center of thestring, this string undergoing a tensile bending after its formation.

[0005] According to another characteristic of the invention, the stringis characterized by the fact that the oblong monofilament appears as acontrasted spiral through the transparent binder.

[0006] A single flat central thread is sufficient for obtaining thedesired effect. Preferably, the central monofilaments are oblongmonofilaments with a very high toughness, whose cross section has ageneral rectangular shape with rounded edges. They constitute a core ofthe string in a way. It is also possible to provide a few smallmonofilaments arranged at the periphery of the string in order to resistabrasion. The central monofilament (or bundle of multifilaments) can becolored in order to increase the luminous effect of iridescence. It isalso possible to color the mass of polyurethane or binder.

[0007] Other characteristics and advantages of the invention will appearin the course of the following description of a particular embodimentgiven only as a nonlimiting example, with regard to the drawings whichrepresent:

[0008]FIG. 1, a cross section of a string according to the presentinvention.

[0009]FIG. 2, a view of a string according to the invention.

[0010] In FIG. 1, it is seen that the string according to the inventionis composed of polyurethane 1, forming the matrix of the string andimpregnating and surrounding the multifilaments and the monofilaments.The polyurethane is found between multifilaments 2 and centralmonofilaments 3 thus forms a homogeneous assembly. It also forms rind 4or exterior envelope layer of the string. Distributed inside of thematrix of multifilaments, in the center of their mass, are oblongmonofilaments 3 which, in the example represented, are two in number.But in all cases, monofilaments 3 are connected with multifilaments 2 bythe polyurethane matrix.

[0011] Monofilaments 3 also take on the dynamic work of the string. Thestring according to the invention, furthermore stiffened by the presenceof monofilaments, is easier to string up.

[0012] Multifilaments 2, as well as monofilaments 3, consist ofpolyamide known under the name “nylon 6.6,” whose shore hardness “D” is85, or other polyamides and copolyamides.

[0013] The process of manufacturing by twisting of such a string isconventional, that is to say, it consists of coating monofilaments andmultifilaments before twisting, and then twisting them with a level oftorsion of 50-200 turns/meter, the string being, after twisting andevaporation of the solvent, run through a die for contour regularizationand evacuation of the excess polyurethane. After which, a tensilebending is exerted on the string which produces delamination and causesthe central monofilaments to appear through the polyurethane.

[0014] The spools of multifilaments, oblong monofilaments and aremounted on a rotating tray on which the threads are coated with asolution of polyurethane. The threads go into a furnace which evaporatesthe solvent and prepolymerizes the polyurethane. Then, the threads arethrown in a ring, and the binder is definitively polymerized in afurnace, after which traction is exerted on the thread in order toobtain the desired effect. The string can thus be constructed in asingle operation without restarts.

[0015] The tensile bending exerted on the string, for example, by meansof a capstan, or by passage over an axle or through a squeezing die,causes delamination which makes the spiral of the central monofilamentsappear as a helix through the transparent polyurethane. The pitch of thespiral is a function of the twisting.

[0016] It has been observed that the strings according to the inventionhave a sound, during impact of a ball, which is similar to that ofnatural gut.

EXAMPLE

[0017] The string represented in FIG. 1 has a diameter of 1.30 mm andhas a breaking strength greater than 80 daN. Its elongation at break ison the order of 25%, and its weight is 1.5 g per meter. It is composedof two 2200 decitex oblong monofilaments which, in the course oftwisting, are spirally wound, and of four multifilament threads,including 140 filaments each whose diameter is 28 microns. The assemblyof the threads is twisted at a rate of 80 turns/meter.

[0018] Although, in the preceding description, the multifilaments andthe monofilaments are made of polyamide, this material can be replacedby any other synthetic material having good dynamic characteristics,such as polyester, for example. The central monofilament threads canhave any cross section provided that they have flat sides so that thecontact between the two threads can occur over a nonlocalized area.

[0019] It goes without saying that numerous variants can be provided,particularly by substitution of technically equivalent means withoutconsequently diverging from the scope of the invention.

1. A synthetic composite string, particularly for a tennis racket,having threads (2) of a first synthetic material and binder (1) made ofa second synthetic material impregnating to the core, connecting andcovering said threads which are spirally wound, the melting point of thebinder being less than that of the threads and its elasticity muchhigher, said threads being made up of multifilaments, characterized bythe fact that at least one monofilament (3) with two flat sides isarranged in the center of the string, this string undergoing a tensilebending which causes delamination.
 2. A composite string according toclaim 1, characterized by the fact that oblong monofilament (3) appearsas a contrasted spiral through the transparent binder and multifilaments(2).
 3. A composite string according to either claim 1 or claim 2,characterized by the fact that monofilaments (3) and multifilaments (2)are made of polyamide, the binder consisting of polyurethane.